1. Field of the Invention
The present invention relates to a plano lens, i.e., a piano lens for use in a rear-projection type projector, a viewing-angle increasing plate of a liquid crystal display device, a plasma display device, an electro-luminescent display device or the like, a light diffusion plate for diffusing a back light for a liquid crystal display, lights of various illumination light sources or the like, and further to a screen for a rear projection type projector.
The present invention relates to a rear-projection type video display apparatus, i.e, a so-called rear-projection type projector.
2. Description of the Related Art
Recently, a projection-type display apparatus for emitting a luminous flux having a polarization characteristic, e.g., one employing a light valve such as a liquid crystal panel or the like has been developed. The projection type display apparatus employing a liquid crystal panel is arranged such that an image light spatially modulated by the liquid crystal panel is enlarged by a projection lens and then projected on a screen. The projection type display apparatus includes a front projection-type and a rear-projection type.
FIG. 1 is a perspective view showing a schematic arrangement of a rear-projection type video display apparatus by which a user watches, from a front side of a screen 2, an image projected on the screen 2 from a rear side of the screen 2. The rear-projection type video display apparatus has a video projector unit 1 for emitting projection video light L, a transmission type screen 2, and a reflection mirror 3 for reflecting the projection video light L from the video projector unit 1 to lead it to the transmission type screen 2.
This transmission type screen 2, i.e., a rear-projection type projector screen is usually formed of a Fresnel lens and a lenticular lens 5 extended in the height direction of the screen 2.
In the rear-projection type video display apparatus having the above arrangement, the Fresnel lens 4 converts the incident projection video light L from the video projector unit 1 into substantially parallel light, and the lenticular lens 5 diffuses the substantially parallel light in the left and right direction (i.e., a width direction of the screen 2).
Thus, the rear-projection type video display apparatus usually enlarges and projects the projection video light L from the video projector unit 1. Specifically, a viewer watches an image obtained from the projected light transmitted through the transmission type screen 2.
However, it is frequently observed that the rear-projection type video display apparatus is generally used in a bright room. In this case, external light such as room illumination light or the like is reflected on a surface of the lenticular lens 5, and a viewer sees the reflected light together with the video light emitted from the screen 2, which lowers the contrast of the image. In order to prevent the contrast of the image from being lowered, the rear-projection type video display apparatus employs a smoke plate (not shown) in front of the lenticular lens 5 to absorb a part of the disturbance light, thereby suppressing the lowering of the contrast.
When the smoke plate is provided as described above, the smoke plate also absorbs a part of the video light transmitted through the smoke plate similarly to the disturbance light, which lowers brightness of the image. In order to increase the brightness, a light source having a larger consumed power is required. This becomes a bar to achieving more efficient power consumption and requires a countermeasure for elimination of heat resulting from the increased consumed power, which leads to a new problem of increasing the manufacturing costs.
A plano lens employing the lenticular lens, a transmission type screen employing such plano lens, i.e., the rear-projection type projector screen, and a rear-projection type video apparatus employing such plano lens and such rear-projection type projector screen have many problems. The problems are as follows.
(1) In the lenticular lens formed by extending a lens element in the upper and lower direction (vertical direction), light is diffused in the horizontal direction, which allows a user to watch an image even from a diagonal direction. However, since light is seldom diffused in the vertical direction perpendicular to the horizontal direction, if the user moves his viewpoint in the vertical direction, the user encounters the disadvantages that a range within which the user can observe a sharp image is extremely narrow.
Specifically, as shown in FIG. 3A, for example, in the rear-projection type video display apparatus employing the lenticular lens, as shown by a line a of FIG. 3A, an area within which of light vertically incident on a plate surface of the lenticular lens is 50% or larger has a shape of an elliptic cone which is flat in the vertical direction. Specifically, if an area within which at its center of light diffused in the horizontal direction is 50% or larger of that obtained at its center is an area having an angle of about 30.degree., then an area within which of light diffused in the vertical direction is 50% or larger of that obtained at its center is an area having an angle of about 20.degree. as shown in FIG. 3C.
(2) Moreover, since the lenticular lens has a precise lens shape on the whole surface, even if a slight defect lies in a part thereof, the whole of the lens can not be used. Therefore, it is necessary to handle the screen with a considerable case. Moreover, recent increase of a picture projected area requires more careful handling of the screen, which inevitably leads to the increased costs.
(3) Since the screen formed by combining the Fresnel lens and the lenticular lens spreads the projected light mainly in the horizontal direction, the screen provides a wide angle view in the horizontal direction. On the other hand, the screen provides only a narrow angle of view in the vertical direction. Therefore, some viewer recognize unevenness of luminance distribution of an image and a partial unevenness thereof and sometimes recognize such uneven luminance as a horizontal light band.
(4) When a black stripe is provided between lens elements of the lenticular lens, the black stripes cannot be formed at an interval narrower than a predetermined one in order to achieve a sufficient effect of the lens, which lowers the contrast of the image and provides a low resolution.
(5) Moreover, since the projected light emitted from the video projector unit 1 or the front-projection type video display apparatus or the like generally presents an illumination distribution in which illumination at a center portion corresponding to a picture angle is bright and illumination gradually becomes dark in the direction toward a periphery side. Therefore, an illumination distribution of an picture presents a sharp curve.
(6) In the screen formed by combining the above Fresnel lens and the lenticular lens, multiple reflection of light is produced between the Fresnel lens and the lenticular lens, which shows a viewer observed images which are overlapped one another.
(7) Moreover, it is sometimes observed that optical interference is produced between the black stripe of the lenticular lens and the projected image and hence a pattern resulting from the interference, i.e., a so-called moire occur in the observed image.
Use of the screen which diffuses light widely, i.e., the screen having a so-called wide diffusion provides only a low gain (luminance in a direction at a certain angle of emission/amount of incident light), i.e., a low luminance but can provide a flat gain curve having less fluctuation with respect to an angle of view. On the other hand, the screen having a strong directivity provides a high gain but the gain is drastically lowered as the angle of view becomes larger. This drastic change of the gain shows that movement of an observation position easily changes brightness of a picture displayed on the screen when a user observes a screen with the naked eyes.
As described in "Characteristics of rear projection screen and measurement method thereof" by Emori, "Optical technology contact" vol. 11, No. 5 (1973 page 17 to page 23, especially page 18, since a human eye has a logarithmic sensitivity with respect to luminance, even if the gain is fluctuated within a substantial range of a peak value which is twice as high as a lowest value, a user recognize only even brightness.
However, it is said that, if the gain is fluctuated beyond a range in which a peak value is three times as high as the lowest gain, then there occurs a so-called hot spot, i.e., hot band phenomenon that a portion corresponding to a peak gain (which is usually located at the center of the screen) appears to be brighter.
According to the above letter, the most suitable screen has a peak gain of 3.5 and a gain obtained at a bend angle (angle of view) of 30.degree. which is higher than 25% of the peak gain.
A significance of measurement of screen performance with a gain at a certain bend angle will be described with reference to FIGS. 4 and 5.
It is assumed that a rear projector is attached to a screen having an aspect ratio of 16:9 and a user observes an image displayed on the screen with the naked eyes being located at a position away from the screen by a distance 3H which is three times as long as a height H of the displayed picture and being in front of a center position of the display picture on the screen. The above distance 3H is considered as a standard observation distance for an NTSC television receiver and a high-definition television (HDTV) receiver.
In this case, when a viewer observes a wide-screen picture having an aspect ratio of 9:16 such as that of the HDTV, as shown in FIG. 4, bend angles in the vertical, horizontal, diagonal directions are 9.5.degree., 16.5.degree. and 18.8.degree. at most, respectively.
Further, when a plurality of viewers watch the screen, as shown in FIG. 5, they watch the screen in front of the screen with being positioned in parallel to the screen, some viewer is positioned at a position which corresponds to a width-direction edge of the screen and is positioned away from the screen by the distance 3H, and watches the screen with the naked eyes being located at the same level as that of the center point of the displayed picture. As a result, as shown in FIG. 5, bend angles in the horizontal and diagonal directions are 30.7.degree. and 31.6.degree. at most, respectively.
Even in the above case, it is necessary to avoid a shading, i.e., a so-called uneven brightness on the screen. In general, even if the shading of 15% to 50% occurs, it is not so problematic when a viewer watches the screen with the naked eyes. However, if the shading of 70% or larger occurs, it is not permissible. An area having the shading of 50% or smaller caused when a viewer watches an image displayed on the screen is called a favorable impression area. If this favorable impression area is enlarged, then a screen area suitable for observation can be enlarged.
In a practical projector, the shading is estimated in consideration of an incident angle of a video light projected on a screen and evenness thereof. However, when the shading resulting only from a screen is estimated, it is possible to such shading in the form of values based on a relationship between a peak gain and a gain obtained at a certain bend angle.
However, recently, as a light projector unit employing an optical space modulation device (i.e., a light valve) such as a thin-film-transistor liquid crystal display or the like has been developed more, a projector increasingly obtains higher optical output year by year. Therefore, there is demanded a screen not only having a first effect of achieving a high peak gain but also having an effect in which some diffusion at the screen enlarges an area suitable for observation.